FAULT IN SAWTOOTH MOUNTAIN RANGE

Big Discovery Because of a Line that 'Stood Out'

Fall 2010 Issue | By Andrew Taylor

While looking at a highly detailed new topographic image of Idaho's Sawtooth Range, Idaho State University geosciences professor Glenn Thackray had an "eureka moment" when he discovered a previously unknown active earthquake fault about 65 miles, as the crow flies, from Boise.

ISU researchers estimate the fault has been active twice in the last 10,000 years, about 4,100 and 7,000 years ago.

The researchers examined a Light Detection and Ranging (LIDAR) image. LIDAR is a remote sensing system used to collect topographic data with aircraft-mounted lasers capable of recording elevation measurements at a rate of 25,000 pulses per second and can have a vertical precision of about six inches. The images can be displayed so they don't show an area's vegetation. Four years ago while doing some research on glaciers in the Sawtooth Range, Thackray was examining a high-resolution, "bare-earth" LIDAR image of the mountains: this is when he noticed a line running through the image in the vicinity of Redfish Lake.

"The black line stood out and I thought that it had to be an earthquake fault," Thackray said. "It was long suspected that there was an active fault in the Sawtooths, but without the LIDAR technology it would have been exceptionally hard to find."

Since that time, ISU researchers have been on the ground documenting the fault that is at least 25 miles long and could be as long as 40 miles. It is located on the eastern edge of the range and comes within five miles from the town of Stanley. A portion of it runs through the upper end of Redfish Lake. It runs along the range from near Stanley Lake to at least as far south as Petitt Lake.

"The reason this discovery is so important is that it is within the heavily visited areas of the Sawtooth National Recreation area, very close to the town of Stanley, and within 65 miles of Idaho's largest city, Boise, and the most populated area in the state," Thackray said. "We would like to know how big the earthquakes are along this fault and how active it is."

Thackray emphasized that the fault is cause for concern, but not alarm, for visitors to and residents of the Stanley-Sawtooth area, and to the residents of the Wood River or Boise valleys. The discovery may have implications for land-use and emergency planning, and perhaps building codes. Depending on the magnitude of a potential earthquake, it could do damage to surrounding areas.

There are few major, active faults in Idaho. Idaho's three other major faults run along the base of the Lost River Range (where the famous Borah Peak magnitude 7.3 earthquake, the largest ever recorded in Idaho, occurred in 1983), the Lemhi Range and the Beaverhead Range.